Daniel and Jorge Explain the UniverseDo gravitational waves leave a mark on the Universe?
Daniel and Jorge talk about whether gravitational waves cause permanent distortions of space.
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Hey Jorge, where were you when you heard about the first gravitational wave discovery?
I don't know, probably home, in my pajamas. That's like ninety nine percent of my life.
Are you telling me you don't remember? How could you forget such a pivotal moment in science history?
Well, I remember pivotal moments in history, just not a physical history. Was this a very significant moment for you?
Oh? Of course it was huge. People have been looking for gravitational ways for decades and I was personally one of the skeptics, so it was mind blowing when they actually saw one.
Because you were proven wrong? Is that the historical event here?
It certainly left an imprint on me, like.
A big footstep in your brain. I am more Hammack cartoonists and the creator of PhD comics.
Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I love to be wrong about physics discoveries, but.
Only physics discoveries. What does your spouse think about that?
I don't have the expertise to be intelligently wrong about anything else. I'm just wrong about other stuff.
I see. There's a difference between being intelligently wrong and regular wrong. But that doesn't seem right.
When it comes to big ideas in physics. I sometimes disagree with the mainstream and think, oh, that will never happen, or that's not going to work, or the universe can't be that way I saw. I'm a bit more skeptical sometimes than others. But then when the universe comes through and delivers an incredible result or discovery, I'm happy that it did.
We could sound like physics is just a bunch of people throwing out random ideas and arguing about it until the universe reveals.
Its Well, you know that's not the complete process, but generating random ideas is part of the process. The next step is sorde of a filter like does that idea make any sense? And can it describe our universe at all? But there is definitely a step where you're just like brainstorming random craziness. Maybe this, maybe that, Maybe it's all just kittens all the way down.
Sounds like the cats mew. But welcome to our podcast Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we do our best not to be wrong about what we do and do not understand about the nature of the universe. One thing we're not skeptical about is humanity's ability to understand the nature of the universe, to cast our brains out into the cosmos, to wiggle it along with gravitational waves, to send it flying into the hearts of neutron stars and deep down into the frothing craziness of quantum mechanics. We will keep pushing and pushing until we understand the entire universe and explain most of it to you.
Yeah, although, Daniel, if you like being wrong so much, why don't you do it more often?
Oh, you'd be surprised. It's not a rare event.
It's almost a hobby. But it is an amazing and wonderful universe. It's so big and so incredibly vast that it kind of makes you wonder what out there can have an impact on it.
If anything, we are definitely still understanding all the ways the universe works, the way energy slashes through, the way particles fly through space, what space even is if you have to even have space in our universe. There are so many very basic questions that we don't have answers to about the very nature of the cosmos in which we reside, and what that means for human existence, what it means to be alive in this crazy cosmos, and you know how we should spend our lives, and so we keep digging into the very firmament of the universe we find ourselves in to try to understand its basic nature. What is this place we find ourselves in?
That's right, because there are a lot of questions we can ask about the universe, and the universe is always happy to provide its interesting answers. The universe is strange and weird and sometimes very unexpected, none more so than some of the amazing discoveries we've made in the last few decades.
Yeah, and we can be surprised by the universe, sort of in two different ways. One way is to come up with a new idea for how the universe works, a theory, a description of the mathematics underlying the universe, and then go out and search for those things. For example, Einstein's general relativity predicted black holes before we even saw them. It suggested these should be a real thing. They should be out there in the universe, they should be happening, you should be able to find them, and people search for them for decades and then finally eventually actually found them, despite a large fraction of the community being skeptical that they existed at all, So those folks were surprised because what was predicted was actually out there.
Well, we found something, but I thought that you told me last time that we're not one hundred percent sure they're black holes.
Oh, you're absolutely right. We're not one hundred percent sure or basically anything we know about the universe, but specifically black holes. We've never been close enough to one and never seen the heart of one to be sure that there's really a general relativistic singularity. There's definitely something there, very compact and very massive, very much distorting space and time. I don't know if it's actually a black hole or a dark star, or a fuzzball or something else quantum mechanical.
That's like you're saying, the only thing we can be sure about is that we're not sure.
Science is a process we hope gradually bending towards the truth. As we refine our mental model for what we think is happening out there in the universe. We can come up with clever ways to test it, and sometimes the universe says, Yep, you were totally right, good job, And sometimes the universe says, nuh uh, something else completely different is going on.
Yeah, and speaking of bending the universe and our minds, another interesting discovery that was made in the last few years that literally has made ripples across the science landscape and the universe itself are gravitational waves. The ripples and the fabric of space itself that are made by super heavy objects moving really really fast.
That's right, And actually gravitational waves are made by everybody. Anything with mass that's accelerating is making a gravitational wave. Hold up your hand right now and wiggle it back and forth and boom, you just created a gravitational wave which is rippling through the.
Universe more like a wave Wavelet maybe when you do it with your arm or what do you call it, like a small wave in a small pond, a ripple.
Yeah, I'm not judging anybody's waves by their size. You know, science doesn't matter when it comes to gravitational waves. But the point is that everything out there that has mass and that accelerates changes the shape of space and ripples that information their existence their gravity throughout the universe in a way very similar to how an electron creates an electric field, and if you wiggle it, it creates a wiggling electric field, which is basically a photon sending its message out there through space. So it's amazing that we understand gravity well enough to talk about how it wiggles and how it ripples, and how space can bend and flex and do all sorts of crazy things.
Yeah, because, as we talked about before, gravity is not like a force something that attracts to things, an actual kind of bending of space. That's how physicists see it. And so when you have things moving through space, they cause ripples in this bending, sort of like moving through molasses or water.
Right exactly. Everything that has mass has a gravitational field, and then if you move that mass, the gravitational field moves, but it doesn't move instantaneously. So the gravitational field that's like one light second away from you doesn't change immediately if you wiggle your mass, but it does change very very close to the mass, and then that change ripples out at the speed of light. So as you move a mass back and forth, that affects the gravitational field, and the information about you having moved it propagates through the gravitational field at the speed of light. That's basically what a gravitational wave is. It's just the information about gravity being updated because something has been accelerated.
Yeah, I feel like it's almost like if you had special gravitational glasses or something that lets you see gravitation waste, you would see them rippling all around you. It'd be almost like a super noisy environment that you're in with gravitational waves being generated by everything and bouncing and a lot don't if they bounce, but being rippled out in all directions by everything.
Yeah, that's a really insightful comment. Because we don't have gravitational glasses, Like, we can't see directly the curvature of space. That's what's really happening. When you create a gravitational field. Really you're bending space around an object to change the path of things. But because we can't see that, you can't look at a chunk of space and say how bent it is. All we can see is the effect of it on stuff. That's why originally we thought gravity was a force because it looks like there's a force because we can't see the curvature directly that's causing it. So really gravity is what we call an apparent force. It's like a force you have to add to our description in order to account for the motion that we see because we didn't understand that it was just due to the curvature of space, because we can't see that directly. As you say, we don't have gravitational glasses. Some of them are really really big enough to register detectors, and some of them very very minute.
Yeah, well, we don't have gravitational glasses, but we do have gravitational ears or gravitational microphones. In recent years, we've been able to set up incredibly large and accurate experiments that can sense these gravitational waves coming to us from space in the rest of the universe, and it was a huge discovery.
It really did rock the world of physics to accomplish this. You know, Einstein predicted gravitational waves would exist, but he also predicted that they would be undetectable, that the effect would be too small for humanity to ever notice. So he was right that they exist, but he was wrong that we couldn't discover them. It was an incredible technological feat really, just like amazing engineering of an experiment to build something sensitive enough to detect this very very slight effect of gravitational waves.
Yeah, I'm always impressed by engineers by anything they.
Do anything that works at least.
Yeah, so they've detected gravitational waves and we're learning more about them. But how much do we know about gravitational waves? Are they everything we think they are or are they maybe weirder than we think they are. So today on the podcast, we'll be asking the question do gravitational waves last forever?
I love the gravitational waves as mind bending in brain rippling as they are continue to provide questions and ideas and new mysteries for us to explore.
Yeah, they bend space and time and our minds at our brains. Well, this is an interesting question, Daniel. Do gravitational waves kind of last forever or at least believe a lasting imprint on the universe? Forever? Forever is kind of a big word or a long word, what's the right gadgett?
It sort of stretches your mind to imagine something lasting forever. But here, because we've described gravitational waves as sort of ripple through space, it's like an update of the gravitational field as it propagates through the universe. Imagine that when the thing that makes the gravitational waves stops wiggling, that the gravitational waves stop. Also that they sort of pass through you and then on to the rest of the universe without leaving any sort of permanent effect.
Yeah, sort of like if you're out in the ocean bobbing on the waves and the waves just kind of go through you and they keep going on after you.
Right.
The question is, does the same happen to gravitational waves.
Or do they leave some sort of lasting mark on space time as they propagate through like footprints in the sand.
Well, ocean waves usually leave a bit of a sea sickness in me that lasts a good bit of time, and.
Then you deposit something over the edge of the boat that leaves a mark in the ocean.
Yes, it's all the cycle of life, or the cycle of the universe. The beautiful, the circle of the universe.
I think that's part of the water cycle. Right.
Well, this is an interesting question. Do gravitational waves leave a lasting imprint on the universe? Do they last forever or do they fade away at some point? As usual, we were wondering how many people out there had thought about this question, had thought about gravitational waves and what they do.
So thanks very much to everybody who volunteers for this segment of the podcast. And you out there who have never volunteered, who have never written in, who haven't heard your own voice on the podcast, we want to hear from you. Write to us two questions at Daniel and Jorgey dot com. It's easy and fun.
Think about it for a second. Do you think gravitational waves leave a lasting imprint on the universe. Here's what people had to say.
My yes is yes, But I don't have a clue how this imprint would look like.
It's an interesting question. Well, if the analogy with a regular wave holds, you know, you'd expect that that will never truly disappeared or will just get smaller and smaller smaller. The question is is there some sort of pixelation or discrete level where they basically disappear or not?
So I don't know to learn about.
Well, the electromagnetic force I believe leaves an imprint on the universe in the form of the cosmic microwave background radiation. So if electromagnetism can do that, I don't see any reason that a different fundamental force like gravity couldn't do the same thing. In this case, leaving an impression on the universe with gravitational waves. So yeah, I definitely think it's possible.
I would imagine possibly indirectly, just with how gravity influences mass in the universe.
But I'm I'm sure all right. Lots of interesting ideas here, going all the way way back to the cosmic microwave background radiation.
Yeah, lots of really good references here also, like the discussion of maybe a minimum level of gravitational waves, like getting into quantum gravity. Very cool stuff.
Yeah, so let's break it down, Daniel, Well, we already talked a lot about what is a gravitational wave. What else can we say about what a gravitational wave is?
I think it's worth exploring how you actually see a gravitational wave. You know, how you like build the detector that can measure a ripple in space and in time because it's a little bit subtle. You know, what we see when a gravitational wave passes is our lengths distorted. If you have it, for example, two big long rulers and they're perpendicular to each other, as a gravitational wave passes, you'll see one of those rulers get shorter and then longer, and the other one then gets shorter and then longer. So you see this sort of like oscillation in the lengths of those arms of your l as it passes through. But there's a wrinkle there because you can only detect it if you build your arms the right way.
Yeah, it's sort of like a distortion of space that passes through you, sort of like in the movies when they try to depict like sound waves or like energy ways, do you see sort of a ripple in the image. That's kind of what's happening. Right, It's like space itself kind of shorts in the tracks in one direction, and.
The sort of mental steps to get there. Remember, are that we have some object out there, like a really massive black hole that's accelerating near another black hole. That's why it's generating the gravitational waves. And we said the gravitational waves are basically an update in the gravitational feel or the gravitational force, right, because as the object that's generating the gravity is moving, its gravitational field is also moving. But remember also that we think about gravity not as a force or having a field to it, but as just bending of space. And so now instead of changing the gravitational field, you're changing the curvature of space itself. Right, that's why we call it a ripple in the fabric of space time, because it's curvature in space that actually is what causes gravity. But how do you actually measure that, right, how do you measure changes in distance of the universe itself? Well, if you just build like a really long stick and you hold it out, you won't measure any gravitational waves because that stick is held together by atoms, which prefer various bond lengths, and so as the gravitational wave passes through, they'll resist a change in its length. It's like sort of too strong. Instead, the way to detect gravitational waves is to use something like beams of light. Instead of having like a long physical ruler, just have like two mirrors at the ends and bounced light back and forth. By measuring how long it takes light to go back and forth, then you can measure how far apart those mirrors are. So gravitational way that's propagating through space will sort of bring those two mirrors closer together and then further apart, and that's what you're actually measuring.
Yeah, it's like having a ruler amane out of empty space. Basically right, like each instead of having a ruler that's a solid object, you just look at space and how long it takes a laser to go through a space and then back again exactly.
It's the same reason that we can't measure the expansion of space or notice the expansion of space very well locally, right, people talk about how space is expanding, Why am I not expanding? Why is the Solar System not expanding? That's because there are forces in play to hold you together. The atoms in your body hold you together even though space is expanding out from under you, and gravity holds the Solar System together even though space is trying to expand it out. And so in the same way, to measure the ripples in space, you need something which isn't being held at a fixed distance, So you need these things just sort of separate it at a certain known distance and then bounce light back and forth and see if the travel time of light changes. There's one more sort of experimental wrinkle there, which is that the changes are so small it's very difficult to measure, So they have to actually send light beams both directions come back and then measure the difference in those travel times by using interference of those photons. So it's like really virtuoso experimental technique. I remember visiting these labs in the late nineties when I was thinking about going to count Tech for grad school, for example, and thinking they're never going to get this to work. Oh my god, this is impossible. But I was very glad when ten years later I was proven wrong.
Yeah, they're amazing experiments. And so let's get into some questions I have about this, like, for example, why doesn't light also get stretched out over this space? And also doesn't the Earth itself count as a fixed ruler, So let's get dig into this and also whether these gravitational waves have a lasting impact on the inverse. First, let's stick a quick break.
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Right, we're making waves with gravitational waves. Are there weirder than we think they are? And do they leave a lasting imprint on the universe? Do they last forever? So we've talked a little bit about what gravitational waves are and a little bit about how we measure them. The super tricky thing because you want to measure a house. Space itself is expanding, but here on Earth things kind of held together by other forces. Like you said, you can really measure the bending of space with a fixed ruler, but it doesn't the Earth also count as a fixed ruder. Like if I have a mirror here on a mirror over there, aren't they held together by the floor.
Yeah, if you attach your mirrors to the Earth, then it's basically just like building a really long ruler. Be very difficult to measure gravitational waves. So when they build their experiment Ligo, they try to isolate those mirrors from anything around them. So if the Earth moves or wiggles or shakes or anything happens, it doesn't affect the location of the mirrors. But this is one limitation of our experiments and why people are hoping to build a new version that's actually out in space that's free from all these effects of Earth's gravity and Earth's bonds and all that kind of stuff. Yeah.
I was going to say, like, isn't the perfect gravitational detector something where it's floating in space, right, so one end is not connected to the other.
End, Yes, exactly. And they have a plan for that project, which they hope is going to launch in about twent d thirty seven. Three spacecraft out in space millions of kilometers away from each other, shooting lasers at each other to measure their relative distances. It sounds like science fiction, but maybe one day it will be true.
Yeah, I mean, what could go wrong? Space ships and lasers. I mean, that's that's the dream, isn't.
It fully operational space ships exactly?
That I need a fully operational science experiment. Make sure the lasers are green.
I was just going to say the lasers would be pretty faint so they wouldn't be damaging, but you know, in order to see them millions of kilometers away, they're actually going to have to be pretty bright. So I hope nobody's eyeballs gets in the path of those lasers.
Yeah, what could go wrong? Well, speaking of lasers, another question I had was, when you're trying to measure this bending of space using light, doesn't light also get bent by the gravitational wave or the bending of space? Doesn't light sort of in a way sort of speed up or slow down?
Yeah, super fascinating question and very confusing and something that I struggle to get my mind around sometimes. Because light is definitely affected by the curvature of space. Right, We know that light gets bent by masses. As it goes by a huge blob of dark matter, for example, it can get lensed, and that is how we see the curvature of space and the change of the direction of light. But remember that nearby light always travels at the speed of light, and that sort of defines what distance is. Actually. Our definition of the meter now is like how far light has traveled in a certain time slice. Because light doesn't change its speed as space gets curved, It only just changes its direction.
Doesn't Gravity also kind of effect time as well, right, Like if you're near a black hole, time slows down and you'd actually kind of see light slowing down.
You're absolutely right, there is a gravitational time dilation effect. If you're in an area with strong curvature, then clocks will run slower. So, for example, you're far away from a black hole and you're looking at somebody who's holding a clock and they're near a black hole, their time will slow down, right. And also if you are watching photons near a black hole, you can see them going on all sorts of crazy speeds. Because there's an ambiguity in how to even define the velocity of things that are far away from you and through some sort of space curvature. We've talked about this a few times on the podcast, that general relativity doesn't even really allow you to define velocities of things that are either very far away if the universe is expanding, or through curved space, because there's sort of multiple ways to bring that thing's velocity vector over to you to measure its velocity. But here we're talking about like very small deviations in space and very local deviations in space, and so from our point of view, we don't expect it to affect the speed of photons.
Well, I'm sure it all works out in the math, but I think The point is that when you're using lasers to measure these distances, as the gravitational wave is passing by, the speed of light sort of remains the same, so you can sort of measure the length of space changed exactly.
And that's really what we're talking about here, is relative distances. You know, we talk about spatial curvature. We don't mean curvature with respect to some external ruler, like your mental picture is probably that misleading bowling ball in a rubber sheet analogy where a mass is bending space into some other dimension. Right, where you have a two dimensional universe and the mass is bending it in some third dimension. Our bending of space that we talk about is intrinsic. It's just a changing of relative distances between things. So a space is curved between two points, it just means that those two points are now closer together. And that's exactly what we're measuring by sending a beam of light through and saying, oh, how long does it take light to get through or measuring that relative distance.
Well, it's super tricky, but humans and namely engineers, have done it. They measured gravitational ways that come from these crazy events out there in the universe. Like two black holes circling each other in a death spiral, and so you can capture the moments right before these things are spinning super fast and crashing into each other. And so that's the kind of event in the universe that makes big gravitational ways maybe the biggest events that we know about. But then the question is, like what happens to gravitational waves. Do they keep rippling out there forever or do they get absorbed by things as they move through the universe?
Right, And your naive picture is probably thinking about like an antenna broadcasting a radio signal. It just sort of propagates out through space, broadcasting a radio signal that's sort of shortened time, like a shout, like a hello, you know in your Hello, broadcasts out through space, and it passes through space and then it fades, and once it's gone, you can't really detect that it was there anymore. That's sort of probably your mental image for a gravitational wave. It creates a ripple in space as the black holes inspiral and then collide, and then that ripple passes through the universe. Maybe it's detected by clever humans and aliens on its path, and then it just passes by them and leaves space unchanged.
But I guess I have a question though. Don't do gravitational waves they hit something, they just pass through. They never get absorbed or even as a earlier bounce.
No, they absolutely do get absorbed, and they can do complicated things like reflect and bounce. I think we talked about that once when we were talking about gravitational waves passing around black holes. They can get lensed, for example, by black holes, and so there's all sorts of interesting wave effects. But as you say, they can also get absorbed. Like what happens when the gravitational wave passes through the Earth is that it's squeezing the Earth a little bit, and then it's squeezing at the other direction, and that does take some energy, and so it's depositing a little bit of energy. It's actually heating up the Earth a little bit. It's like a little tidal squeeze. So that helps the gravitational wave fade. Right. First of all, it's fading because distances are increasing. It's spreading out through a larger and larger distance, and so it goes like one over distance squared. But also when it passes through matter, it does deposit a little bit of its energy into that matter.
Yeah, and the universe is full of matter, right, I mean not a huge amount. But like in the viewer, to emit a great ravitation a wave in the middle of the Milky wave, for example, it would have to go through all of those stars in the Milky wave before it can go out to the rest of the universe.
Yeah, that's absolutely true. But now think about a simpler scenario. Imagine you just have like two particles floating freely in space and they're exactly one light second apart. A gravitational wave passes through, and it sort of brings them closer together and then further apart again. When the gravitational wave is done, does it leave them as the same distance as they were originally or is there some sort of permanent distortion there.
That's the question, because I guess gravitational wave isn't just like something that stretches space. It kind of contracts and stretches space, right, That's kind of what a wave is. It's like it's an up and a down.
M Absolutely it is. And gravity is really complicated stuff. It's much more complicated than electromagnetism, for example, because it interacts with itself. You send a photon out through space, that photon flies through space. It's a ripple and the electromagnetic field. But photons don't talk to us other photons and don't create other photons. Like two photons, as we talked about on the podcast once, don't directly interact with each other. They can do it indirectly via other virtual particles, but photons themselves don't bounce off other photons. That's not true for gravity. Everything that has mass or energy creates gravity and influences everything else with mass and energy. And gravitational waves have energy, which means they create gravitational waves, and then those create gravitational waves, and those create gravitational waves. So you have this sort of like nonlinear effect where gravitational waves are constantly spewing off other gravitational waves.
Yeah, so it sounds like gravitational waves do sort of leave a pretty obvious lasting impact on things, Right, Like if a gravitational wave was big and it went through Earth, and like you said, it squeezed Earth in one direction and then stretched it in the other direction, and then did the vice versa in the went through, which does heat up the Earth a little bit which lasts for a while at least, right, Like, it depods some energy and we keep that energy and you can measure that energy.
Yeah, that's certainly true. And there's this other effect even for isolated stuff, you know, even for two particles floating out in space, there's something called the gravitational wave memory effect. It's like it changes space as it passes through and leaves it changed. Right, those two particles floating out in space, when they get wiggled together, there's no energy deposited, like when the Earth gets squeezed, because there's no like bond between these two particles floating out in space, and yet still there's a permanent effect on those particles. This again is called the gravitational wave memory effect. It's like footprints in the sand. Once the gravitational wave passes through, it changes things even after it's gone.
Wait wait, wait, what's it called again.
The gravitational wave memory effect.
Okay, sorry, I'd forgot for a second.
I walked right into that. It's called the podcast or dementia effect.
Well, I think what you're saying is that we know that the gravitational waves leave an impact in stuff around the universe, right, like if it goes through stuff, it leaves a little bit of energy because it had to squeeze it and stretch it. But now I think maybe the question we're really asking in this podcast is do gravitational waves leave an imprint on like space itself? Like the space itself get scarred or marred or you know, imprinted by the gravitational wave.
You make it sound so negative, like it's been vandalized by gravitational waves or something like it's been ruined, Like, man, I built this space and then those crazy teenagers marred it.
Well, I mean space is so pristine and beautiful. Yeah, you put a bunch of ripples in it. You are kind of changing the aesthetics.
Yeah, but you know space is also filled with gravitational waves. So the space we started with has already been imprinted on by all the gravitational waves that came before us.
Okay, so then the question we're really asking is do gravitational waves leave a lasting imprint on space itself? And you're saying the scenario we should be imagining is like to particles out there in space floating at a certain distance from each other, and they don't interact any other way at all. Like, there's no electromagnetic forces between them, there's a I mean there's gravity. Can they have gravity between them?
Well, they're far enough apart that this essentially no gravity.
Yeah, okay, essentially no gravity, but no weak force, no strong force. They're just like two lonely particles out there in space. And then gravitational waves comes through. Does it change the space between them permanently in a way that you could be like, hey, something came through here.
Yeah, really fun question. And in the seventies, people who were playing with the equations of general relativity and doing calculations discovered to their surprise that it should. And this is fun because it's like a theoretical surprise. You know, we have equations for Einstein's general relativity, but we don't always know the consequences of them. Sometimes when you sit down to say what would happened in this scenario, you run into something unexpected because to understand the effects on the universe of these equations you have to do some calculations. You have to set it up and say, I'll let me see if I can predict this scenario. And so this was discovered in the seventies and then a lot of progress has been made in the last few decades, but all sorts of various different kinds of memory effects.
What's it call again?
I forgot a memorable name, fool me once.
This effect has a name, it's called the effect. And so to say, it's like a theoretical thing that says that space does get kind of altered permanently when a gravitation wave moves through. And so how does that work? How does this effect work? How does it come up in the equation?
So there's actually a few different effects. There's a nonlinear one and the linear one. I think the most interesting and least confusing to understand is the non linear effect. And this comes up because gravity, as I was saying before, couples to itself, like gravity creates more gravity, whereas photons don't create more photons. Gravitational waves generate gravitational waves as they pass through space, and so this creates this sort of like nonlinear effect because the energy doesn't just fly through the universe. Its sort of like deposits itself in space itself as it goes along. It creates these little mini gravitational waves that change space.
Wait, what as it goes through stuff or even in empty space.
Even in empty space, right, Gravitational waves themselves contain energy.
And so part of their energy goes into making other gravitational waves as it goes along.
Yeah, exactly, And so that's one of the ways that they fade. And so if you look at like the prediction for what should happen to two particles as a gravitational wave passes through them, the sort of canonical prediction you're familiar with from like Ligo et cetera, is that they get further and closer and further and closer, and then they get back to their original location. If you include all these effects of like gravitational waves generating more gravitational waves, you see that they don't come back to where they started. That there's a lasting effect on the distance between these two particles, which basically means space itself has been stretched permanently.
Interesting, but I guess it's a picture you're painting for us here, is that the gravitational wave generates more waves. But then don't those waves also fade away eventually? Where does the permanence come from?
The permanence comes from this nonlinearity. Right, they're fading, but they're also generating new gravitational waves and generating new gravitational waves.
But each time it's smaller, isn't it?
Each time it's smaller. But you know, it's an infinite series. An infinite series. Sometimes they'd ever, sometimes they go to zero. In this case, they add up to a constant. They add up to a non zero value.
Oh, I see it. It's sort of like a permanent echo. Like if you're in a closer room and you say hello, hello, hellooo, that hello never kind of goes away, and it stays where the gravitational wave went through. So it's sort of like it it leads a permanent echo wherever it goes.
Yeah, exactly, It's just like a permanent echo. And so that's really kind of interesting because it suggests the gravitational radiation is fundamentally different from other kinds of radiation. Right, it's not as capable of transmitting energy, because, as you say, it deposits some of that in space as it goes along.
Interesting, So then that's what it does to space. And where do there are two little lonely particles come in? How do those two particles notice that space space is now full of gravitational echo.
Because they don't go back to their original distance. If they started out one light second apart before the gravitation the wave has passed through, then they wiggled out and in and out and in a little bit. But when the gravitational wave has passed, they're now like one point zero zero one light seconds apart, whereas they used to be one light second apart. So you can measure this, you can say, oh, my gravitational waves passed by, yet my particles are still further apart than when they started.
M you mean, like the preminent echo that lingers after the wave goes through actually results in kind of stretching space between them exactly.
And that's what gravity is, right, is the stretching or compression of space. And so it's like made more space. It's like deposits some of that energy into creating new space between these two test particles, a very very tiny amount. Remember, gravitational wave effects are very very subtle, which is why they are so hard to see. The original experiment, Lego had these arms where the mirrors were like kilometers apart, and they saw the distance between those mirrors changed by one to one thousands of the width of a proton. Right, These are really, really tiny effects, and the gravitational wave memory effect is even smaller than the gravitational wave effect itself.
Wait wait, wait, what's the gravitational memory effect?
I forgot again.
I'm just kidding. So, like, if you measured how far these two lonely particles were before out in space, and then with gravitational wave went through, and then you measured it again, you would measure them to be a little bit further apart than before.
Exactly if there was nothing else influencing them, no gravity, no bonds, nothing else but just the shape of space, then yes, they would permanently be further apart than when they started, even long after the gravitational wave has passed through them.
And it's like a positive stretching effect. It's not a compression effect.
It's a positive stretching effect. It deposits energy, creates new space.
Interesting, and this is happening all over the universe. So are you saying gravitational waves like black holes crashing into each other? Is they're part of the reason the universe is expanding? Or does it contract in some places and expands in other places? Right, Like, when the gravitational waves get generated, does it compress space?
That's a really interesting question whether it overall, like integrated over all of space, contributes to the expansion of space or whether it cancels out somewhere. I'm not one hundred percent sure of the answer, but I think that this is only a positive contribution to the shape of space. And again, this is really a tiny effect, almost impossible to measure, much much smaller than the expansion of space due to dark energy. But I think it would technically contribute to the expansion of space. Yes, that if you had like a universe where nothing was moving, so no gravitational waves were created, versus a universe where things were swashing around and gravitational waves were being made, that second one would be expanding faster than the first one.
Well, I wonder if maybe, like because that energy has to sort of come from somewhere, right, like when the black holes get formed when they swirl around each other, And wonder if that has an effect to shrink the local space, but then it stretches everything else out.
Yeah, the energy, remember, comes from the masses of the objects that generated. When two black holes merge, you have like one of them is fifty solar masses, an others fifty solar masses. The black hole that they result in is not one hundred solar masses, it's like eighty because they've generated an incredible amount of gravitational radiation, like twenty solar masses worth. So that's where the energy comes from.
All right, Well, let's get into how you might measure this interesting effect and also what it all means about our understanding of gravity and the universe. But first, let's take another quick break.
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We're talking about what we're talking about, Daniel.
We're talking about moving you to retirement homes.
We're talking about the memory effect of gravitational waves. This idea that gravitational waves, as they propagate through the universe, they have a lasting effect on space itself. It's stretching space, depositing little, tiny and everlasting echoes of gravitational energy which stretch space and make it bigger. And so you're saying, Daniel, this effect is super duper duper small. How can we even measure this? Can we prove that this theory is right?
So it is a theory currently. It's a consequence of general relativity and one we've seen in the math, but we've never actually seen it out there in the universe. Gravitational waves we have seen. We know those are real. But this gravitational memory effect that we keep forgetting what it's called, this part has not yet been seen. It's a theoretical prediction. And remember that we have great confidence in general relativity because it's been such a virtuos description of how the universe works and the nature of space itself. But we also think that it's probably flawed because it can't describe the quantum mechanical nature of the universe. So not everything that general relativity predicts is guaranteed to be true, which is one reason why we want to go out and test this. But your right, it's also much much smaller effect than gravitational waves. It's predicted to be like twenty to fifty times smaller than current gravitational waves effects we have measured. We don't think that our current experiments like LEGO are going to be capable of seeing this very easily.
Mmmmm Right, because, like we talked about before, Logo is a ruler which is sort of attached to itself. It is a solid ruler in a way. Right.
Well, Logo does a really good job of trying to be independent from the Earth as much as possible, but this is something that the Earth has that Logo just cannot escape, and that's the Earth's gravity. Right. We have these two mirrors where light is bouncing back and forth between them. Imagine they're getting like squeezed a little bit closer together or a little bit further apart. And even if the gravitational wave wants to pass through and leave them a little bit further apart the Earth, this gravitational field will sort of pull them back, right, It will pull them down and prevent them from staying a little bit further apart. These mirrors are like suspended on cables, right, So imagine like a pendulum that's been left a little bit away from equilibrium. If there's gravity there, it'll just swing right back to the equilibrium position. So the Earth's gravity sort of erases this gravitational memory effect in Lego.
M saying the Earth remembers. But I guess that doesn't make a lot of sense to me, Like why would the Earth care how far apart these mirrors are.
Well, I don't think the Earth has an opinion, Like it doesn't matter to the Earth at all. But the Earth does have gravity, and the effect of gravity will be to pull these things back to where they were.
In what way does Earth's gravity pull their mirrors back? Like, why would the Earth want to put them back in the same position? What was special about the original position?
Well, the Earth itself has these powerful bonds, and so we don't think the Earth itself is changed, right, So the Earth still has the original gravity that it had before. And these mirrors are not floating in zero gravity, right. Their original position was determined by the gravity of the Earth. So they're going to end up back in that same position if you don't apply some forces to them. It's sort of like if you went and pushed on one of those mirrors, what would happen, Well, it would swing in one direction, but then gravity would bring it back, right, but.
Only if you push it against Earth's gravity, Like if you push it perpendicular or to the side gravity, the Earth's puls is the same, isn't it, Like the Earth is just pulling them towards the center of the Earth.
Yeah, so the mirrors are all being pulled down, right, all being pulled down towards the center of the Earth. And so if you give them a nudge away from that line towards the center of the Earth, they're going to naturally trend back towards the center of the Earth.
But if I push it along a circle around the Earth, the gravity is the same, isn't it.
Well, So remember these mirrors are suspended on a cable with a fixed length, right, and so if you push it then it's basically moving it up, just like if you have a ball on a string and you push on the ball, the ball goes up because the string is a fixed length, and so now the Earth is going to pull it back down to the equilibrium position.
Oh, as you were saying, the way Ligo is designed, these things are on pendulums, and so the Earth's gravity wouldn't let you have a permanent change in the space between them.
Yeah, that's exactly right, And in principle you might be able to observe it before Earth brings some sort of back to the equilibrium position. There's like a little bit of information there. The memory effect might last for a little while before the Earth erases it. But logos really just not set up to make this kind of measurement. Instead, what you need is the same kind of detector, but where everything is in free fall right, where there is no nearby massive object pulling on everything. Basically you need this thing out in space.
I see. So there's just more of an excuse to have space lasers, the operational space lasers.
That's really it's just the long con When these guys in the seventies were coming up with these Kayglier, they were just like, dot dot dot space lasers. How do we get there?
Yes, that's every physicist dream, isn't it. Shoot lasers in space?
Space lasers. It's pretty cool, though, space lasers. I mean, that's pretty awesome. You got to say, right, that's going.
To Yeah, Reagan thought that too.
We're just going to shoot them back and forth between these quiet little detectors in space. I promise you, we're not going to shoot our eye out, mom, if you like that kid in a Christmas story.
Okay, So then the idea is that you would have these detectors out there in space, you know, kind of an empty space, and then you would measure the distance between them with super duper high accuracy. And then you would not just measure the wave as it washes by, right, but you might be able to measure like, oh, after the wave went through. Now we're a little bit further apart, which proves this idea of the.
What gravitational way of memory? I'm only saying that because maybe the listeners forgot, and I want them to be confused.
The memory effect of gravitational waves right, that's the idea, right, that.
Is the idea. And so there is actually a design for this. It's not just physicists dreaming up space lasers. It's called LISA, the Laser Interferometer Space Antenna, and it's basically three spacecraft out there in a triangle. And you know, LIGO is a few kilometers long. They're bouncing light back and forth. This thing is going to be millions of kilometers long.
What so these are like space space?
Oh yeah, these things? I mean, why not make them really far apart, right if you can? Because the further apart they are, the more sensitive you are to really small gravitational waves. Not only could this potentially detect the memory effect, it could also measure the gravitational background effect. Like, as you say, every time you move your arm, you're creating gravitational waves, and everything that's out there in space orbiting is creating gravitational waves. We could sort of measure this sort of background gravitational wave noise of the universe and get some information about that using LISA.
Oh wow, because I was thinking this was more like maybe the gravitation will prob be satellites that did some relativit experiments around the Earth. This is like out there in space base, like out there between the planets, right if you're talking about millions of kilometers or is it still around Earth's orbit.
It's going to be orbiting the Sun sort of with the Earth. And you know, millions of kilometers sounds really long, and it is pretty big. It's bigger than the Earth, which is kind of awesome. But these things will sort of stay near the Earth so that we can talk to them, communicate them, control them. They'll be orbiting the Sun sort of with the Earth like.
A fully operational space station.
Not yet fully operational. This thing is just in the planning stages. I think the earliest targeted launch date is twenty thirty seven, which is like comfortably far away enough in the future that nobody has to like actually build anything today. So nobody's actually started constructing anything.
I see. We're still in the prequels. We're like in the Clone Wars or Rebel or a Rogue one.
We're still drawing pictures of Death Star on chalkboards at this point in the story. We're not actually building any but you know, if we build it, we will learn so much about gravitational waves gravitational memory, gravitational background. Maybe also even see gravitational waves from the Big Bang. You know, ripples in space that were created in the very very early moments of the universe.
Wait, what, so it's be so accurate and so sensitive to gravitational waves that you would measure these ripples from the beginning of the universe like those are still around.
Those we think are still around, sort of the same way that like the cosmic microwave background radiation is still around. These are photons from the very early universe, but they don't go all the way back to the very beginning of the universe. They only go back as far as the universe has been transparent. Before some moment in the early universe it was opaque, so you generated photons, they just got absorbed. Those photons are not around anymore. The oldest photons we can see are back when the universe was transparent to light. But gravitational waves can go through almost anything, right, So the universe is basically transparent to gravitational waves, which means that if we could detect very very faint gravitational waves, we could see all the way back to well before that moment when the universe was opaque and see signals and get information about the very very early universe. So Lisa would be really powerful sort of astrophysical and cosmologically.
Yeah, you could hear like the Big Bang itself, like the Bang, right, that's the idea.
Yeah, we might see ripples from inflation. You know, that would be really awesome.
All right, So we're building these awesome space telescopes and we might measure and confirm that gravitational waves do have this memory effect that leaves kind of a standing echo across space. What would that mean about our understanding of the universe.
It would mean that once again, general relativity is awesome and accurate. It might also help us solve some puzzles we have about like blacklack holes. Remember that one mystery about black holes is like where does the information go when something falls into a black hole, because we think that like black holes don't release any information about what's inside them. On the other hand, we also think black holes evaporate eventually due to hawking radiation, and so that information has to go somewhere, but we don't really understand it. It's possible that if gravitational waves are leaving permanent imprints on space, then maybe things falling into a black hole are like changing the space around a black hole in some important way, leaving their information there as they fall into the black hole, so that it isn't actually destroyed.
Hmmm. Interesting, Like as it falls in it leaves a little bit of a graffiti in space itself before it gets destroyed by the black hole.
Yeah. Or maybe it's like a space angel, you know, think about it in a positive way. It's like making its mark on space.
Space angel.
You know, you can make snow angels or sand angels. Can you lie down in space and like wag your arms and make a space angel? Oh?
I see, I thought you were talking about like actual space angels. I was like, Eh, that's an interesting idea for space epic.
No, that would be a spiritual angel. I'm talking about a real physical thing, you know. I guess in principle, if you stand around and wave your arms, you are making gravitational waves in the shape of a space angel. I suppose.
Yeah, and it technically it is permanent, like you are distorting space forever.
Yeah, you are, so be careful, everybody.
What if I walk around in a pattern that says Kilroy was here? Am I marring the universe.
Some future alien society will build a very rare sensitive gravitational wave detector and they will read your message and they will wonder why did he choose to send that? What does that mean?
Didn't he have anything better to do? All right, Well, I think that answers the question. Gravitational waves do leave an imprint on the universe, on the stuff that it passes through, and maybe theoretic it does also leave an imprint on space itself, something that lasts forever, that echoes throughout eternity, that maybe somedays some alien species or maybe us in the future can read and learn about what happened.
Yep, if we're still doing this podcast in fifteen or twenty years, then maybe we'll have an episode talking about the discovery of gravitational wave memory.
Assuming we remember this episode, which I'm guessing probably not. All right, Well, we hope you enjoyed that. Thanks for joining us, See you next time.
Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. How is US dairy tackling greenhouse gases? Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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